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How does the new coronavirus compare to influenza?

11th Mar 2020

We were asked by readers how the Covid-19 and SARS-CoV-2, the coronavirus that causes it, compares to influenza, in terms of both its infection rate and its death rate.

It is difficult to get a precise infection or death rate for either illness. Covid-19 is a new disease, and data on the number of infections is imperfect. The number of influenza cases, meanwhile, can vary a great deal year-to-year, depending on various factors including which of the many strains of the virus are spreading. However, the World Health Organisation has attempted to compare the two.

Covid-19 could be deadlier, and more likely to put someone in hospital

Based on current evidence, Covid-19 has the potential to kill more people and cause more hospitalisations than seasonal influenzas. However, it hasn’t done so yet, because so far it hasn’t spread as widely as seasonal flu.

Seasonal flu is still a major killer, which contributes to the rise in death rates during colder months, and to the overloading of medical systems in winter. Dr Suzanna McDonald, national programme lead for influenza at Public Health England, warned: “Whilst the flu is common, it can be deadly for older adults, very young children and people with underlying health conditions.”

The WHO estimates that between 290,000 and 650,000 respiratory deaths globally each year are associated with seasonal influenza.

Public Health England estimates that on average 17,000 people have died from the flu in England annually between 2014/15 and 2018/19. However, the yearly deaths vary widely from a high of 28,330 in 2014/15 to a low of 1,692 in 2018/19. Public Health England does not publish a mortality rate for the flu.

The USA’s Centers for Disease Control and Prevention (CDC) does publish estimates of the number of flu cases annually, including estimates for the number of hospitalisations and the number of deaths. The number of cases has ranged from nine million to 45 million a year, and the number of deaths ranged from 12,000 to 61,000. These figures suggest that over the past decade in the United States, the percentage of flu cases that resulted in hospitalisation has consistently been between 1% and 2%, and the percentage of cases that resulted in death has been between 0.1% and 0.2%.

According to the WHO, current evidence on the new coronavirus suggests 15% of all illnesses are severe infections, requiring oxygen, and 5% are critical infections requiring ventilation. These levels are “higher than what is observed for influenza infection”.

Mortality rates also appear to be higher for the new coronavirus, although the WHO does warn that the “true mortality of Covid-19 will take some time to fully understand”. Based on current data, the crude mortality rate (the number of reported deaths divided by the reported cases) is between 3-4%—although the actual rate, when including mild cases where people do not get diagnosed, is likely to be lower. Globally, for seasonal influenza, the WHO estimates the mortality rate is usually below 0.1%.

However, the WHO also notes that mortality is “to a large extent determined by access to and quality of healthcare”.

Influenza can spread faster, but this coronavirus may infect more people

In its analysis, the WHO says that both the time from infection to the appearance of symptoms, and the time between successive cases is shorter for the influenza virus, meaning that it is able to spread quicker than the new coronavirus.

It also says that a “major driver of transmission” for influenza is transmission in the first three to five days of the illness or even before symptoms have appeared, while this does not yet appear to be the case with the new coronavirus.

The analysis says each person with Covid-19 infects an average of between two and 2.5 other people with the virus, which is higher than the rate for influenza, but it notes that direct comparisons are difficult because the estimates are “very context and time specific”.

The most at-risk groups are also slightly different between the two illnesses. Current understanding of Covid-19 suggests that elderly people and those with underlying illnesses have the biggest risk of developing a severe infection, while those most at-risk from influenza also include children and pregnant women. Initial evidence suggests that, while children are important drivers of transmitting influenza, they are less affected than adults by Covid-19.

There is no vaccine for this coronavirus

While there are vaccines and anti-viral medications available to combat influenza, no vaccine currently exists for this new coronavirus. The WHO says clinical trials are ongoing and there are more than 20 vaccines in development for Covid-19, but none have yet been licensed.

And because the various strains of influenza have mostly been circulating for many years, plenty of people have developed a degree of immunity to them. That’s not the case for this coronavirus.

Symptoms

The NHS lists the symptoms of Covid-19 as a cough, high temperature and shortness of breath. The WHO says the most common symptoms are a dry cough, fever and tiredness, but some people may also experience aches and pains, nasal congestion, a runny nose, sore throat or diarrhoea. Symptoms tend to emerge around five days after catching the virus. We’ve written more about this here.

New advice published on 18 May now says that anyone experiencing loss of taste or smell (known as 'anosmia') should also self-isolate.

According to the NHS, flu symptoms can come on very quickly and include a sudden fever, aching body, feeling tired or exhausted, a dry cough, a sore throat, a headache, difficulty sleeping, loss of appetite, diarrhoea or tummy pain, feeling or being sick. Children can also get pain in the ear.

Update 18 May 2020

This article was updated to reflect new government guidance about symptoms including a loss of taste or smell.

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